The most widely accepted technique of manufacturing extruded vulcanized type cables, at present, is to feed the conductor through a series of extruder heads and apply concentrically the semiconducting, and insulating compounds. The cables are subsequently cured in a saturated steam environment followed by cooling under pressure. Protective coverings are applied in subsequent operations.
In the steam curing process, the insulated conductor is moved through the vulcanizer and exposed to pressurized (typically 250 psi) saturated steam followed by cooling under pressurized water (typically 250 psi). The thermosetting compound contains curing agents which are activated at the high curing temperatures. The speed of reaction depending on the exposure temperature (for steam, approximately 210.degree. C).
The steam curing process is being utilized in catenary and in vertical installations. The vertical configuration is advantageous from the processing point of view, because it is easier to achieve cable core concentricity. However, because of the very high cost of building towers for this type of curing system, the length of the curing pipes has to be limited and hence, low processing speeds have to be utilized. Long length curing pipes are desirable because polyethylene and ethylene propylene rubber insulations are characterized by having high thermal resistances and thus, heavily insulated cables, as used for high voltage operation, take a long time to cure. The catenary installation uses longer vulcanizing pipes, typically 300 to 600 ft. long. Even at these lengths, commercial 69 and 138 kV cables, having 650 and 800 mils of XLPE insulation, are manufactured at a typical rate of about 2 to 5 ft/min.
An additional shortcoming of the steam curing method of manufacture is the fact that during the time it takes for the crosslinking reaction to take place, the outside of the insulation is exposed to high pressure steam. It has been widely demonstrated that steam penetrates into the insulation and creates microscopic cavities, imposing limits on the dielectric strength of the cured insulation.
A further shortcoming of the catenary installations is the fact that the touchdown of the cable occurs in the pipe when the insulating compound is still soft and deformable. Because of this, certain degree of out-of-roundness becomes inherent with this method of production. The deformation is aggravated by the large size pipes used and is accentuated for the higher voltage cables, which generally have greater weights. Hence, the catenary installation can be utilized for manufacture of cables having a limited weight only.
Large lengths of extruded insulated cables made by the steam curing process have been in operation for many years. Their ability to withstand the high operational voltages results from the heavy insulation walls used on such cables. Reduction in the size of micro-cavities present in the insulation and the elimination of significant deformation of the cable insulation, in accordance with this invention, permits the use of thinner insulation walls and makes possible the manufacture of cables rated for higher voltages without proportionate increase in the thickness of the insulation.
In order to improve the characteristics of the dielectric and the manufacturing process of extruded type cables, a number of different systems have been proposed in the past. The most significant ones relate to curing the cable in a pressurized inert hot gas environment using similar curing tubes to those used in the steam curing process (as described in IEEE Power Apparatus and Systems 94, March/April 1975, and in the IEEE Conference Record, 1974, Underground Transmission Conference, Dallas, Apr. 1974. Another expedient has been the extruding of the insulation on the conductor through a long sizing passage in which the insulation is simultaneously cured (U.S. Pat. No. 3,054,152, Sept. 18, 1962).
This last method for curing thermosetting compounds was first disclosed in U.S. Pat. No. 2,742,669 Apr. 24, 1956). Some further refinements of the method are disclosed in U.S. Pat. No. 3,868,436 (Feb. 25, 1975); and in U.S. Pat. No. 3,928,525 (Dec. 23, 1975).
The long land die curing system in the above-described references requires a very precise control of the compound output from the extruder. If the compound output is smaller than the output required to fully fill the long land die, then only a portion of the cable core will be in contact with the die. The other portion of the surface will not follow the circular configuration of the die and will be rough. This roughness will contribute to a lower breakdown of the insulation. In the case of excessive output of compound from the heat, an excessively high pressure will build up in the head and in the long land die. The excessive pressure will prevent the flow of the lubricant and can result in a push-back of the compound through the tip of the insulating head. This condition also leads to lower dielectric breakdown of the cable insulation and in extreme cases will force an interruption of the cable manufacturing process. Precise control of compound output from the insulating head, as required by the long land die curing system is difficult and in many cases not practical.
A further shortcoming of this method of curing is the required use of long land dies having lengths of 50 to 100 ft., for achieving processing speeds comparable to those used in catenary installations.
The novel curing system of the present invention can be used in a process that extrudes three layers, as in U.S. Pat. No. 3,446,838; or four or five layers, as disclosed in U.S. Pat. No. 3,885,058. With the present invention, using a short land die, there is a composite mini-curing pipe larger in diameter than the short land die and adjacent to the discharge end of the short land die. The mini-curing pipe consists of two sections: the first one made of insulating material and incorporating a heat booster, and the second one consisting of a heated metal pipe with a length selected in accordance with the desired processing speed. Both sections of the pipe have the same inside diameter. A liquid curing medium is introduced into the mini-pipe close to the end of the short land die and is forced out at the far end of the mini-curing pipe. The heating of the cable core is accomplished in various ways which will be explained in connection with the drawing. The heat booster preferably comprises a coil connected to a high-frequency generator which sets up a high-frequency field for generating heat in the metal conductor which contacts with the inside surface of the insulation system and also in strips of metal which heat the outside of the insulation across a liquid medium.
The mini-curing pipe contains a liquid curing medium which contacts with the outside of the insulation system and which is at a temperature much higher than the temperature of the steam previously used for curing cable insulation. The liquid curing medium should be of high molecular weight, at least 800, to minimize its penetration into the cable core. The liquid is circulated by means of a pressurizing circulating pump.
This novel curing process has the following advantages:
a) It allows for the curing of the cable core at pressures much higher than pressures utilized in other known methods, and in this way significantly minimizing the size of cavities in the insulating system.
b) It allows for the maintenance of high processing speed using curing pipes with lengths much shorter than that of other known curing systems.
c) It assures minimum out-of-roundness, because the diameter of the mini-curing pipe is not substantially larger than the diameter of the cable core.
d) It does not require tight tolerances in the extruder output of compound, because from this point of view, tolerances between the diameter of the cable core and of the curing pipe are substantial.
e) It provides an insulation system with a minimum penetration of liquid into the core, because of the use of high molecular weight liquid curing mediums.
f) It provides a compacted structure of the core, with smooth surface, which is obtained in a short land die without use of lubricants.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.